The present invention relates to an electric double layer capacitor of high electrostatic capacitance which utilizes the principles of an electric double layer.
Recently, high-capacitance capacitors based on the electric double layer principles have been developed as backup power supplies for memories in electronic systems, and are widely used with microcomputers and IC memories.
One type of electric double layer capacitor is disclosed in U.S. Pat. No. 3,536,963, for example. FIG. 4 of the accompanying drawings shows the structure of the disclosed electric double layer capacitor. The electric double layer capacitor comprises a single basic cell composed of a pair of current collectors 20 of an electron conductor which serves as a pair of collector electrodes, a pair of carbon electrodes 10 made of activated carbon particles, a nonconductive gasket 30, and a separator 40 positioned between the carbon electrodes 10 for preventing electrons from moving between the carbon electrodes 10. The carbon electrodes 10 are made as paste electrodes from a concentrated slurry which is a mixture of powdery or particulate activated carbon and an electrolyte. The electrolyte has three functions to perform. It serves as a promoter of ion conductivity, an ion source, and a binder for the carbon particles.
It is important that the internal resistance of an electric double layer capacitor be low. The internal resistance of an electric double layer capacitor is greatly affected by the contact resistance of active carbon of the polarized electrodes and the contact resistance between the collector electrodes and the polarized electrodes.
Therefore, in order to reduce the internal resistance of the polarized electrodes and the contact resistance between the collector and polarized electrodes, each basic cell should be kept under vertical pressure to bring the particles of the paste activated carbon into good electric contact with each other. Conventional electric double layer capacitors require each cell to be kept under a pressure of about 100 kg/cm.sup.2, though it depends on the size of the electrodes, the size of the particles of the carbon material, or the kind of the electrolyte used. In prior electric double layer capacitors, the cells are kept under pressure by deforming the outer cases of the capacitors or bonding the current collectors strongly to gaskets. If an electric double layer capacitor is to be used as a large-capacitance capacitor, e.g., a power supply for energizing a motor, then it is necessary to increase the cross-sectional areas of the electrodes of the basic cell. Therefore, the pressure to be applied to the basic cell has to be increased. Increasing the pressure, however, causes some practical problems such as the selection of means for applying the pressure and the need for high rigidity for the outer cell which houses the basic cell.